Method for the preparation of silicon single crystal

ABSTRACT

An improved method is proposed for the preparation of a semiconductor silicon single crystal of N-type by the Czochralski process, which is free from the problem of occurrence of delayed OSFs as defects in the single crystal even after prolonged storage at room temperature based on the discovery that presence of a certain amount of aluminum in the melt of silicon contained in a fused silica glass crucible acts to suppress occurrence of delayed OSFs as a type of defects in the single crystal while copper as an impurity acts adversely in this regard. With a known fact that an about 30 μm thick inner surface layer of the crucible is melted down into the silicon melt during the single crystal pulling-up process, namely, the invention proposes use of a crucible of which the inner surface layer of 30 μm thickness contains aluminum in an average concentration of 40 to 500 ppm by weight while the content of copper is as low as possible not to exceed 0.5 ppb by weight. Alternatively, when the fused silica glass crucible is deficient in the content of aluminum, an amount of aluminum is introduced as a dopant into the melt of silicon in the crucible to supplement the content of aluminum in order to be sufficient to suppress delayed OSFs.

BACKGROUND OF THE INVENTION

The present invention relates to an improvement in the method for thepreparation of a semiconductor silicon single crystal or, in particular,N-type semiconductor silicon single crystal by the Czochralski methodand a fused silica glass crucible used therefor.

As is well known, semiconductor silicon single crystals are mostlyprepared by the so-called Czochralski method or pulling-up method inwhich a silicon single crystal as growing on the lower end of a seedcrystal is pulled up from a melt of high-purity silicon formed bymelting polycrystalline silicon in a crucible of fused silica glass. Oneof the problems in the silicon single crystal prepared by this crystalgrowing method is that so-called "oxidation-induced stacking fault",referred to as OSF hereinafter, are sometimes found therein. These OSFsare very detrimental in the subsequent manufacturing process ofintegrated circuit devices and the like from wafers of the siliconsingle crystal resulting in a great decrease in the yield of acceptableproducts of integrated circuits and the like so that it is eagerlydesired to develop a method by which silicon single crystals absolutelyfree from occurrence of OSFs can be grown.

As an approach to accomplish a decrease in the density of OSFs to asignificant extent, proposals have been made in recent years for the useof polycrystalline silicon having a still higher purity than heretoforeas well as materials of the structure of the pulling-up furnace havingan extremely high purity, improvements in the conditions for crystalgrowing and use of a fused silica glass crucible having the highestpurity available.

In connection with the above described problem due to occurrence ofOSFs, an interesting phenomenon has come to the attention of the artisanin an N-type silicon single crystal, which is a semiconductor silicondoped with a dopant of the Vth-Group elements in the Periodic Table suchas phosphorus, antimony, arsenic and the like, that, when a siliconsingle crystal absolutely free from OSFs by the inspection immediatelyafter completion of the crystal growing is kept standing as such, i.e.in the form of a single crystal rod, for a certain length of time or,for example, for one month or longer at room temperature, a large numberof OSFs are sometimes detected by the subsequent inspection. Thisphenomenon is referred to as the delayed OSF hereinafter. Though notwell understood, it is a presumable mechanism of this phenomenon ofdelayed OSF that the impurities contained in the starting single crystalrod in a very trace amount cause diffusion through the single crystalrod during storage at room temperature to form aggregates which serve asthe nuclei for the occurrence of OSFs. As a consequence of thepresumption of the mechanism, proposals have been made by the inventorsin Japanese Patent Kokai 5-58800 to avoid the phenomenon of delayed OSFaccording to which the single crystal rods are stored at a temperatureas low as possible to retard diffusion of impurities or the singlecrystal rods are sliced into wafers as early as possible so as to blockthe diffusion.

The above proposed methods, however, are nothing more than a method forthe prevention or retardation of the delayed OSFs in a silicon singlecrystal having inherency toward occurrence of OSFs sooner or laterproviding no fundamental solution of the problem if not to mention thesecondary problems unavoidable therein that the low-temperature storageof silicon single crystal rods is a very expensive way to significantlyincrease the production costs and the early slicing of the singlecrystal rods into wafers is accompanied by great difficulties in theproduction planning and storage of the wafers as sliced. Therefore, acomplete solution of the OSF problem can be obtained only by theestablishment of a method for the preparation of a silicon singlecrystal which is free not only from occurrence of OSFs immediately aftersingle crystal growing but also from the delayed OSFs even after aprolonged storage of the single crystal rods as grown at roomtemperature.

SUMMARY OF THE INVENTION

The present invention accordingly has an object, in view of the abovedescribed problems in the prior art, to provide a means for thepreparation of a semiconductor silicon single crystal, in particular, ofthe N-type which is free not only from occurrence of OSFs immediatelyafter single crystal growing but also from the delayed OSFs even after aprolonged storage of single crystal rods as grown at room temperature.According to the invention, this object of the invention can beaccomplished by the control of the distribution profile of specificimpurities within the walls of the fused silica glass crucible in whichpolycrystalline silicon is melted to form a melt from which a siliconsingle crystal rod is pulled up by the Czochralski method.

Thus, the present invention provides an improvement in the method forthe preparation of a semiconductor silicon single crystal by theCzochralski method, in which a silicon single crystal rod is pulled upon the lower end of a seed crystal from a melt of silicon prepared bymelting polycrystalline silicon in a fused silica glass crucible, theimprovement comprising use of a fused silica glass crucible having suchdistribution profiles of aluminum and copper as impurities in thedirection of the wall thickness of the crucible that the averageconcentration of aluminum is in the range from 40 to 500 ppm by weightin the surface layer of 30 μm thickness from the inner surface of thecrucible and not exceeding 40 ppm by weight within the layer adjacent tothe surface layer in the depth of from 30 μm to 1 mm from the innersurface of the crucible and the average concentration of copper does notexceed 0.5 ppb by weight within the layer from the inner surface to theouter surface of the crucible.

The invention proposes a further improvement in the above describedCzochralski process for growing of a silicon single crystal, whichcomprises using a fused silica glass crucible containing aluminum andcopper as impurities in concentrations lower than 40 ppm by weight andnot exceeding 0.5 ppb by weight, respectively, and adding aluminum as adopant to the melt of silicon in the crucible in such an amount that thetotal amount of the added dopant aluminum and the aluminum impuritycontained in the inner surface layer of the crucible of 30 μm thicknessfrom the inner surface is equal to the amount obtained by multiplyingthe amount of the fused silica glass of the 30 μm thick inner surfacelayer by the aluminum concentration in the range from 40 ppm by weightto 500 ppm by weight.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a correlation diagram showing the relationship between theaverage concentration of copper in fused silica glass crucibles and theOSF density in a silicon single crystal grown therefrom after 4 weeksstorage.

FIG. 2 is a correlation diagram showing the relationship between theaverage concentration of aluminum in the inner surface layer of fusedsilica glass crucibles and the OSF density in a silicon single crystalgrown therefrom after 4 weeks storage.

FIG. 3 is a correlation diagram showing the relationship between theaverage concentration of copper and average surface concentration ofaluminum in fused silica glass crucibles in grouping for absence andoccurrence of OSFs in silicon single crystals without or with OSFs grownfrom the crucible and stored for 4 weeks.

FIG. 4 is a schematic illustration of the four blocks 1 to 4 taken froma silicon single crystal by dividing for the test of OSF.

FIG. 5 is a graph showing appearance of delayed OSFs in the siliconsingle crystals prepared in Comparative Examples 1 and 2 as a functionof the storage period of the blocks.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With an object to provide a solution for the above described problems,the inventors have first directed their attention to the sources fromwhich the metallic impurities taken into the silicon single crystalsgrown by the Czochralski method originate. While extensiveinvestigations have been undertaken heretofore for various sources ofpossibility from which the metallic impurities originate, it is anunderstanding in recent years that, in view of the accomplishment of theextremely high purity not only in the polycrystalline silicon as thestarting material but also materials constructing the Czochralskifurnace, what should be investigated as a major impurity source is theimpurities contained, even though the concentration thereof is extremelylow, in the fused silica glass crucible used in the Czochralski methodsince the inner surface layer of the crucible is more or less melteddown into the melt of silicon. As a consequence of the detailedinvestigations on the relationship between the impurity contents in thefused silica glass crucible and occurrence of the delayed OSFs in asilicon single crystal grown by using the crucible, following criteriahave been found that:

1) a positive correlation is found between the average concentration ofcopper in the crucible and occurrence of OSFs in the silicon singlecrystal grown therefrom so that the average concentration of copper inthe crucible must be 0.5 ppb by weight or lower;

2) aluminum acts as an indispensable constituent in the melt of siliconfor suppression of OSFs so that it is not a sufficient condition that acertain amount of aluminum is contained in the crucible as a whole but acertain amount of aluminum must be melted down into the melt of siliconat an early stage of the single crystal growing process by theCzochralski method; and

3) quite satisfactory results can be obtained only as a synergisticeffect of these requirements for the contents of copper and aluminum.

As to the influences of the other impurity elements than copper andaluminum, as investigated so far, no correlation has been found betweenthe content of each element and occurrence of delayed OSFs provided thatthe concentration of the impurity element in the crucible does notexceed 1 ppm by weight as is the case in most of the fused silica glasscrucibles currently under practical use.

The above described findings have led the inventors to the establishmentof the present invention, of which the basic scope consists in that: 1)the average concentration of copper as an impurity in the fused quartzglass crucible as a whole must be as low as possible; and 2) theconcentration of aluminum must have such a distribution profile that theconcentration thereof is high in the vicinity of the inner surface ofthe crucible walls coming into contact with the melt of silicon duringthe single crystal growing process, the concentration of aluminumelsewhere being low. The uniqueness of the present invention consists inthe discovery that a fused silica glass crucible of an ultimately highpurity is not a requirement in respect of the delayed OSF at leastrelative to the content of aluminum.

In particular, the requirement for the distribution profile of aluminumand copper in the direction of the wall thickness in a fused silicaglass crucible is that the average concentration of aluminum is in therange from 40 to 500 ppm by weight in the surface layer of 30 μmthickness from the inner surface of the crucible walls and not exceeding40 ppm by weight within the layer adjacent to the surface layer in adepth of from 30 μm to 1 mm from the inner surface of the crucible andthe average concentration of copper does not exceed 0.5 ppb by weightwithin the layer from the inner surface to the outer surface of thecrucible.

Alternatively to the use of the above specified fused silica glasscrucible in the preparation of a silicon single crystal by theCzochralski method, the invention also proposes a method in which themelt of silicon contained in a fused silica glass crucible, which has avery low concentration of aluminum in the inner surface layer, is dopedwith aluminum in such an amount as to be equivalent to the amount whichcould be taken into the melt assuming that the distribution profile ofaluminum in the fused silica glass crucible be according to the abovedescribed requirement.

When this alternative method is undertaken, the amount of aluminumdopant W, which should be equal to the amount melted down into the meltof silicon from a crucible containing aluminum in the inner surfacelayer, to be introduced into the melt of silicon in analuminum-deficient crucible, of which the concentration of aluminum inthe inner surface layer is lower than 40 ppm by weight, can be estimatedby calculation by taking into account the following equation:

    W=A×t×ρ×C,

in which W is the amount of aluminum to be introduced into the melt ofsilicon from an aluminum-containing crucible; A is the area of thesurface on which the melt of silicon is in contact with the cruciblewalls; t is the thickness of the layer which is melted in the singlecrystal growing process; ρ is the density of the fused silica glasscrucible; and C is the average concentration of aluminum in the layer ofthe crucible walls to be melted down into the melt during the process.Namely, the amount of the aluminum dopant to be added to the melt is thedifference between the value W when C in the above given equation is 40to 500 ppm by weight and the value of W when C is the actualconcentration of aluminum in the inner surface layer of the crucible.

In the following, the present invention is described in more detail.

It should be noted that the impurity contents in conventional fusedsilica glass crucibles currently under use in the Czochralski processfor the single crystal growing of semiconductor silicon are so low foreach of impurity elements that occurrence of OSFs in the silicon singlecrystal at least as grown can be almost completely prevented. Forexample, the average concentration of aluminum and copper are 12 ppm byweight or less and 0.5 ppb by weight or less, respectively.Nevertheless, the phenomenon of delayed OSF frequently takes place inthe silicon single crystals grown by using such a high-purity fusedsilica glass crucible. To the contrary to the generally acceptedunderstanding therefore that the reason for the delayed OSF must besought in other factors than the impurity contents in the fused silicaglass crucible, the unique and novel discovery leading to the presentinvention is that, while the content of copper in the crucible should beas low as possible, the average concentration of aluminum in thecrucible walls must not be so low as in the conventional fused silicaglass crucibles but must be at a certain level within the surface layerof the crucible walls in order to fully prevent the undesirablephenomenon of delayed OSF, in particular, in the N-type silicon singlecrystals grown from the crucible.

FIG. 1 of the accompanying drawing is a correlation diagram between theaverage concentration of copper in ppb by weight within the body of thefused silica glass crucible and the density of OSFs found in the siliconsingle crystals grown by using the respective crucibles after storagefor 4 weeks at room temperature. As is clear from this diagram, apositive correlation is found between these two parameters so that theconcentration of copper must be as low as possible in order to minimizeoccurrence of delayed OSFs although no critical copper concentration canbe given since, even when the copper concentration is 0.5 ppb by weightor lower, occurrence of delayed OSFs cannot be completely prevented.

FIG. 2, on the other hand, is a correlation diagram between the averageconcentration of aluminum in ppm by weight within the 30 μm thick innersurface layer of the fused silica glass crucible and the density of OSFsfound in the silicon single crystals grown by using the respectivecrucibles after storage for 4 weeks at room temperature. As is clearfrom this diagram, a negative correlation is found between these twoparameters.

The experimental results leading to the above described correlationstudies has been further developed to the correlation study between theconcentration of copper in the bulk of the crucible and theconcentration of aluminum in the inner surface layer of the cruciblegiving the correlation diagram shown in FIG. 3 in which the plots offilled and open circles correspond to those silicon single crystalswhich suffered and did not suffer, respectively, from occurrence ofdelayed OSFs. This result has led to a conclusion that the averageconcentration of copper in the bulk of the fused silica glass cruciblemust not exceed 0.5 ppb by weight while the concentration of aluminum inthe inner surface layer of the crucible must be at least 40 ppm byweight in order to almost completely prevent occurrence of the delayedOSF in the single crystal of silicon grown from the crucible.

The above obtained conclusion means that an N-type silicon singlecrystal which is absolutely free from occurrence of delayed OSFs couldbe obtained when the content of copper impurity in the N-type singlecrystal be ultimately decreased so as to avoid diffusion and aggregationthereof in the single crystal while the concentration of aluminum, whichsupposedly has an effect of suppressing the delayed OSF, in the singlecrystal be kept at a certain level.

Accordingly, the present invention proposes the use of a fused silicaglass crucible which, instead of having a uniformly decreasedconcentration of each impurity element, has such a specific distributionprofile of the impurity concentrations relative to copper and aluminumthat the average concentration of copper throughout the crucible body isas low as possible or, desirably, 0.5 ppb by weight or lower while theconcentration of aluminum is kept at a certain level at least in theinner surface layer of the crucible walls. It is noted here thataluminum acts as a P-type dopant in the silicon single crystals so thatthe content of aluminum in an N-type silicon single crystal must belimited not to exceed a certain upper limit since otherwise difficultiesare caused in the control of the resistivity of the semiconductorsilicon. This consideration has led to the completion of the presentinvention according to which the fused silica glass crucible should havesuch a distribution profile of aluminum in the direction of wallthickness of the crucible that the average concentration of aluminum isin the range from 40 to 500 ppm by weight or, preferable, from 50 to 150ppm by weight in the surface layer of 30 μm thickness from the innersurface of the crucible walls and not exceeding 40 ppm by weight or,preferably, 10 ppm by weight within the layer adjacent to the surfacelayer in a depth of from 30 μm to 1 mm from the inner surface of thecrucible.

As to the impurity elements other than copper and aluminum in the fusedsilica glass crucible, no definite correlation has been found betweenthe concentration of each impurity element and occurrence of delayedOSFs in so far as the impurity concentration is within the range in theconventional fused silica glass crucibles so that it makes no matterwhether the concentrations of these impurity elements are conventionalor somewhat higher although it is a desirable way to use a cruciblehaving a purity as high as possible also relative to these impurityelements other than copper and aluminum.

As is mentioned above, the concentration of copper in question is theaverage concentration throughout the body of the fused silica glasscrucible while the concentration of aluminum must be controlled by wayof the distribution profile relative to the inner surface layer of thecrucible walls which is lost by melting down into the melt of siliconcontained in the crucible up to the moment of completion of the singlecrystal growing process by the Czochralski method. This differentiationbetween copper and aluminum is due to the difference in the mobility ofthe respective impurity elements within the fused silica glass. Namely,aluminum is relatively immobile not to cause diffusion in the silicaglass during the single crystal growing process so that the distributionprofile thereof is unchanged throughout the process while copper has arelatively high mobility in the silica glass to cause diffusion so thatcontrol of the distribution profile has no particular significance dueto the change before the start and after completion of the singlecrystal growing process. Namely, it is important that aluminum isconcentrated by segregation in the inner surface layer of the cruciblewhile the concentration of the copper impurity is kept as low aspossible as an average value for the whole crucible body (see JapanesePatent Kokai 4-108683).

Following is a description of the procedure for the preparation of afused silica glass crucible having the above mentioned uniquedistribution profile of the impurity concentrations relative to copperand aluminum according to the present invention.

As is disclosed in Japanese Patent Kokai 63-166791, it is the prior artwhen a fused silica glass crucible from natural quartz having anultra-high purity is desired that the aluminum impurity deposited oraccumulated on the inner surface of the crucible during fusion of thecrucible is removed. In the absence of understanding of the exactmechanism for the segregation of aluminum, this is the only possible wayto ensure a high purity relative to aluminum because of thenon-availability of any effective means to prevent accumulation ofaluminum. Such a procedure to subsequently remove the impurityaccumulated on the inner surface of the fused silica glass crucible isgenerally applicable to other impurity elements to cause surfacesegregation in the manufacture of fused silica glass crucibles fromnatural quartz as the starting material.

The inventors conducted two test procedures for the Czochralski growingof N-type silicon single crystals by using, one, a fused silica glasscrucible prepared from natural quartz and subsequently purified to anultimately high purity by the above described method and, the other, acrucible of synthetic silica having a still higher purity. To thecontrary to the expected results, these silicon single crystals werenothing better or rather worse than the silicon single crystals grownfrom conventional fused silica glass crucibles in respect of occurrenceof delayed OSFs. The investigations undertaken to find the mechanismleading to this unexpected phenomenon have led to a conclusion that thealuminum impurity deposited and accumulated on the inner surface of thecrucible walls acts rather suppressingly on the occurrence of delayedOSFs so that the aluminum impurity on the inner surface of the cruciblewalls should not have been completely removed in order to minimizedelayed OSFs. Assuming that the overall amount of the aluminum impuritybe identical, it is not sufficient that the aluminum is distributedevenly throughout the crucible walls but it is essential that thealuminum impurity is contained mostly within the layer of about 30 μmthickness from the inner surface of the crucible walls so as to betransferred into the melt of silicon before the start of the singlecrystal pulling-up process from the melt. As the other impurity elementsin the fused silica glass crucibles, no particular adverse influenceshave been found thereby on the occurrence of delayed OSFs excepting forcopper of which the concentration must not exceed 0.5 ppb by weight asan average over the whole body of the crucible in order to preventoccurrence of delayed OSFs in an N-type silicon single crystal grownfrom the crucible.

In the manufacturing process of a fused silica glass crucible fromnatural quartz, a powder of natural quartz is first washed with amixture of hydrofluoric acid and nitric acid and then subjected to aheat treatment at 1000° C. to 1300° C. in an atmosphere containinghydrogen chloride and chlorine in combination followed by a second acidwashing with hydrofluoric acid so that the content of copper in thepowder is decreased to 0.3 ppb by weight or lower after the treatmentsto give a high-purity quartz powder. It is known that these treatmentshave no effect to decrease the content of the IIIB-Group elements on thePeriodic Table such as aluminum so that the concentration of aluminumof, e.g., 12 ppm by weight before the treatments is retained as suchafter the treatments. Alkali metals, iron and nickel are among thoseimpurity elements which can be particularly removed from the quartzpowder by these treatments.

The thus purified quartz powder is deposited on the wall of a moldrotating around the vertical axis and the deposition of the powder isheated from inside to effect fusion and vitrification into the form of afused silica glass crucible. When the temperature on the inner surfaceof the thus molded crucible is about 2200° C., it has been found thatgradual accumulation of aluminum takes place so that the averageconcentration of aluminum in the inner surface layer of 30 μm thicknessreaches about 50 ppm by weight. If the heating is continued at 2300° C.for 5 minutes or longer, a further increase of the aluminumconcentration gradually proceeds so that the average concentration ofaluminum within the inner surface layer reaches about 300 ppm by weight.Prolongedly continued heating at this temperature to exceed 30 minutesresults in an increase in the concentration of aluminum in the layer at40 μm or deeper from the inner surface of the crucible due to thediffusion of aluminum so that the crucible thus prepared cannot meet theobject of the present invention. It is of course that the temperature ofthe fusion and vitrification and length of time therefor in themanufacture of the crucible should be adequately selected depending onthe dimensions of the crucibles because the required amount of surfacesegregation of aluminum depends on the amount of charge ofpolycrystalline silicon into the crucible.

According to the scope of the present invention, it is important that acertain amount of aluminum is taken into the melt of silicon before thestart of the single crystal pulling-up process and subsequently nosubstantial increase is caused in the concentration of aluminum in themelt. This is the reason for the requirement that the necessary amountof aluminum should be accumulated within the 30 μm thick layer on theinner surface of the crucible walls because the thickness of the layerof the crucible walls which is lost by melting down into the melt ofsilicon before the start of the single crystal growing process rarelyexceeds 30 μm. It is of course optional that the aluminum-segregationlayer has a thickness smaller than 30 μm if it is confined within the 30μm thick layer from the inner surface of the crucible although aremedial means can be undertaken even if the aluminum-segregation layeris not confined within the 30 μm thick layer from the inner surface, forexample, by extending the time for keeping the melt of silicon in thecrucible before the start of the single crystal pulling-up procedurebecause, in essence, the requirement is that the necessary amount ofaluminum has been taken into the melt of silicon before the start of thesingle crystal pulling-up procedure.

When the fused silica glass crucible in itself has no segregation layerof aluminum on the inner surface or the average concentration ofaluminum is lower than 40 ppm by weight in the inner surface layer of 30μm thickness as is the case in a crucible prepared from natural quartzand having the aluminum-segregation layer completely removed or acrucible of a synthetic silica glass inherently containing only anextremely small amount of aluminum, it is necessary according to theinvention that the inner surface of the crucible is provided with adeposition of a coating layer doped with aluminum in an amount to meetthe requirement for the prevention of the phenomenon of delayed OSF inthe silicon single crystal grown from the crucible. The thickness ofthis additional aluminum-doped coating layer is about 5 μm in order notto cause falling from the crucible surface. Needless to say, the amountof aluminum contained in the inner surface layer of the crucible isadequately adjusted so as to ensure a necessary concentration ofaluminum in the melt of silicon because the proportion of the volume ofthe silicon melt contained in the crucible and the area of the cruciblesurface coming into contact with the melt depends on the dimensions ofthe crucible. Doping of the fused silica glass crucible with aluminumcan be conducted by the method of putting an aluminum dopant material onthe bottom of the crucible before the crucible is charged with blocks ofpolycrystalline silicon or by the method in which an aluminum dopant isintroduced into the melt of silicon in the Czochralski furnace by meansof a specific instrument. Incidentally, the fused silica glass cruciblehaving the specified distribution profiles of the concentrations ofaluminum and copper according to the present invention can be usedwithout any detrimental effects in the Czochralski growing of a P-typesilicon single crystal.

The amount of the aluminum dopant to be used in the above mentioneddoping method should be equivalent to that of aluminum which should betaken into the melt when the fused silica glass crucible assumedly hasan inner surface layer containing aluminum in the specifiedconcentration. Thus, the amount W of the dopant aluminum is given bytaking into account the following equation:

    W=A×t×ρ×C,

in which W is the amount of aluminum introduced into the melt of siliconfrom an aluminum-containing crucible; A is the area of the surface onwhich the melt of silicon is in contact with the crucible walls; t isthe thickness of the layer which is melted down in the single crystalgrowing process; ρ is the density of the fused silica glass crucible;and C is the average concentration of aluminum in the layer of thecrucible walls to be melted down into the melt during the process.Namely, the amount of the dopant aluminum to be added is the differencebetween the amount W when C is in the range from 40 to 500 ppm by weightand the amount of W when C is the actual concentration of aluminum inthe inner surface layer.

In the following, the present invention is described in more detail byway of examples and comparative examples showing some typical cases.

EXAMPLE 1

A fused silica glass crucible, referred to as the crucible Ahereinafter, having an inner diameter of 18 inches was prepared. Thecrucible A was prepared from a high-purity powder of natural quartz,after a heat treatment at 1200° C. in an atmosphere of hydrogen chlorideand chlorine, by fusion and vitrification at 2300° C. so as to have asegregation layer of aluminum on the inner surface. Table 1 below showsthe analytical results of the contents of 9 impurity elements includingaluminum and copper. The concentration of aluminum is given there in twoways of, one, for the concentration in the surface layer of 30 μmthickness from the inner surface of the crucible, referred to as thesurface concentration in the table, and, the other, for theconcentration in the layer adjacent thereto in a depth of 30 μm to 1 mmfrom the inner surface, referred to as the bulk concentration in thetable, while the contents of the other impurity elements are given as anaverage from the whole volume of the crucible. The units of thenumerical values in the table are ppmw (ppm by weight) for aluminum andppbw (ppb by weight) for the other impurity elements.

The analytical method for the determination of the concentration ofaluminum was as follows. For the determination of the surfaceconcentration of aluminum, a 30 μm thick surface layer was dissolved outwith a 38% hydrofluoric acid and this acid solution was subjected to theanalysis by the atomic absorption spectrophotometric method to determinethe relative intensities of light absorption for aluminum and siliconand the concentration of aluminum was obtained by calculation from therelative intensities for a known concentration proportion. Thereafter,the surface layer having a thickness of 1 mm as measured from theinitial surface before the above mentioned acid treatment was taken bycutting, of which the bulk concentration of aluminum was determined alsoby the method of the atomic absorption spectrophotometry.

Using the above described crucible A, a crystal growing test wasundertaken according to a standard procedure of Czochralski method inwhich the crucible was charged with 60 kg of polycrystalline siliconblocks to form a melt by heating, from which a single crystal siliconrod of 6 inches diameter doped with phosphorus and weighing 45 kg waspulled up in the crystallographic orientation of <100> and subjected tothe examination of occurrence of OSFs either as grown or after storagefor weeks. Incidentally, the polycrystalline silicon as well as thematerials of the Czochralski furnace were all of the highestpurity-grades available in order to minimize the influences by theforeign impurities not originating in the crucible.

In conducting the test for OSFs, four blocks 1 to 4 were taken from thethus obtained silicon single crystal rod by dividing according to theschematic illustration in FIG. 4 and each of the blocks was subjected tothe test of OSFs after the respective treatment described below.

The block 1 was, immediately after dividing the rod into blocks, slicedinto 2 mm thick wafers, each of which was mirror-polished and subjectedto a heat treatment in a schedule including a temperature elevationstage from 800° C. up to 1200° C. at a rate of 10° C./minute,constant-temperature stage at 1200° C. in a wet-oxygen condition for 100minutes, temperature lowering stage down to 800° C. at a rate of 1.5°C./minute and cooling down stage to room temperature. Thereafter, theoxidized surface film on the wafer surface was removed by usinghydrofluoric acid and the wafers were subjected to SECCO etching (F.Secco et al., Journal of Electrochemical Society, volume 119, page 984,1972) for 2 minutes and the density of the OSFs was determined using anoptical microscope.

The blocks 2, 3 and 4 were kept standing for storage at 23° C. for 2weeks, 4 weeks and 6 weeks, respectively, before slicing into 2 mm thickwafers, each of which was subjected to mirror-polishing and the sametreatment and test of OSFs as in the block 1. The results of the OSFtest were that absolutely no OSFs were detected in any of the waferstaken not only from the block I but also from the blocks 2 to 5indicating absence of the phenomenon of delayed OSF.

EXAMPLE 2

The experimental procedure was substantially the same as in Example 1excepting for the use of a fused silica glass crucible, referred to asthe crucible B hereinafter, which was prepared from synthetic silica andcoated on the inner surface with a layer of aluminum-doped silica glassby the sol-gel method. The contents of the respective impurity elementsin the crucible B are shown in Table 1. Absolutely no OSFs were detectednot only in the silicon wafers taken from the block 1 as grown but alsoin the wafers taken from the blocks 2 to 4 after storage at roomtemperature for up to 6 weeks.

COMPARATIVE EXAMPLE 1

The experimental procedure was substantially the same as in Example 1excepting for the use of a fused silica glass crucible, referred to asthe crucible C hereinafter, which was prepared from conventional naturalquartz and contained 1.0 ppb by weight of copper. The contents of therespective impurity elements in the crucible C are shown in Table 1. Themicroscopic examination for OSFs indicated that, although no OSFs weredetected in the wafers taken from the block 1, the density of OSFs wasincreased in the wafers taken from the blocks 2 to 4 stored at roomtemperature for up to 6 weeks as is shown in FIG. 5 by the curve I.

COMPARATIVE EXAMPLE 2

The experimental procedure was substantially the same as in Example 1excepting for the use of a fused silica glass crucible, referred to asthe crucible D hereinafter, which was prepared from high-puritysynthetic silica. The contents of the respective impurity elements inthe crucible D are shown in Table 1. The microscopic examination forOSFs indicated that, although no OSFs were detected in the wafers takenfrom the block 1, the density of OSFs was increased in the wafers takenfrom the blocks 2 to 5 stored at room temperature for up to 6 weeks asis shown in FIG. 5 by the curve II.

EXAMPLE 3

The same experimental procedure as in Comparative Example 2 describedabove was repeated excepting doping of the silicon melt in the cruciblewith aluminum by introducing 4 mg of a powder of high-purity aluminummetal containing 2 ppm by weight of silicon and 2 ppm by weight ofcopper into the crucible together with 60 kg of the polycrystallinesilicon blocks so that the doping concentration of aluminum in thesilicon melt was about 0.07 ppm by weight.

The above mentioned doping amount of aluminum, i.e. 4 mg, was a resultof the estimation by calculation in the following manner using theequation:

    W=A×t×ρ×C,

given before assuming A=3218.82 cm² for a 18-inch crucible containing 55kg of silicon melt, t=30 μm, ρ=2.3 g/cm³ and C=40 ppm by weight or 500ppm by weight to give a result of W=0.89 mg or 11.13 mg, respectively,so that the actual doping amount of aluminum should be somewhere betweenthese two limiting values.

The results of the test for the density of OSFs in the single crystal ofsilicon grown from the thus aluminum-doped melt were that absolutely noOSFs were detected not only in the wafers taken from the block 1 butalso in the wafers taken from the blocks 2 to 5 stored at roomtemperature for up to 6 weeks indicating no delayed OSFs.

                  TABLE 1                                                         ______________________________________                                        Crucible     A        B        C      D                                       ______________________________________                                        Aluminum (ppmw)                                                               surface      102      80       250    <1                                      bulk         5        <1       6      <1                                      Copper       0.5      0.3      1.0    0.4                                     Boron        <200     <200     <200   <200                                    Nickel       <2       <2       <2     <2                                      Iron         <200     <200     <200   <200                                    Chromium     <20      <20      <20    <20                                     Sodium       <100     <100     <100   <100                                    Potassium    <100     <100     <100   <100                                    Lithium      <300     <300     <300   <300                                                         (ppbw)                                                   ______________________________________                                    

What is claimed is:
 1. In a method for the preparation of asemiconductor silicon single crystal by the Czochralski method, in whicha silicon single crystal rod is pulled up on the lower end of a seedcrystal from a melt of silicon prepared by melting polycrystallinesilicon in a fused silica glass crucible, the improvement whichcomprises using a fused silica glass crucible having such distributionprofiles of aluminum and copper as impurities in the direction of thewall thickness of the crucible that an average concentration of aluminumis in the range from 40 to 500 ppm by weight in a surface layer of 30 μmthickness from an inner surface of the crucible and not exceeding 40 ppmby weight within the layer adjacent to the surface layer in a depth offrom 30 μm to 1 mm from the inner surface of the crucible and theconcentration of copper does not exceed 0.5 ppb by weight within thelayer from the inner surface to the outer surface of the crucible. 2.The improvement as claimed in claim 1 in which the average concentrationof aluminum is in the range from 50 to 150 ppm by weight in the surfacelayer of 30 μm thickness from the inner surface of the crucible and notexceeding 10 ppm by weight within the layer adjacent to the surfacelayer in a depth of from 30 μm to 1 mm from the inner surface of thecrucible.
 3. In a method for the preparation of a semiconductor siliconsingle crystal by the Czochralski method, in which a silicon singlecrystal rod is pulled up on the lower end of a seed crystal from a meltof silicon prepared by melting polycrystalline silicon in a fused silicaglass crucible, the improvement which comprises using a fused silicaglass crucible containing aluminum and copper as impurities inconcentrations lower than 40 ppm by weight and not exceeding 0.5 ppb byweight, respectively, and adding aluminum as a dopant to the melt ofsilicon in the crucible in such an amount that the total amount of theadded dopant aluminum and the aluminum impurity contained in an innersurface layer of the crucible of 30 μm thickness from the inner surfaceis equal to the amount of the fused silica glass forming the 30 μm thickinner surface layer of the crucible multiplied by the concentration ofaluminum in the range from 40 ppm by weight to 500 ppm by weight.